1. Field of the Invention
This invention relates to a ceramic circuit substrate having a low resistance surface conductor layer formed on the surface thereof and a method of fabricating the ceramic circuit substrate.
2. Description of the Prior Art
Ceramic substrates have a higher heat resistance and a smaller coefficient of thermal expansion than resinous printed circuit boards. Furthermore, fine wiring patterns can be formed on the ceramic substrates more easily than on the resinous printed circuit boards. For these reasons, the ceramic substrates have widely been used as substrates for various types of ICs which have recently been required to be small in size, to be operated on a high-frequency power, and to be operated at high speeds. These requirements for ICs have recently been intensified. A dielectric constant of the ceramic material and the resistance of the surface conductor layer formed on the surface of the ceramic substrate need to reduced in order that the above-described requirements may be met.
A conducting sectional area of the surface conductor layer of the ceramic substrate tends to be reduced due to fining the wiring patterns, and a sheet resistance thereof tends to be increased accordingly. To prevent the increase in the sheet resistance, the prior art has proposed thickening the surface conductor layer to thereby increase the conducting sectional area so that a low resistance surface conductor layer is provided.
Screen printing which yields a high productivity is generally used to fabricate the surface conductor layer. However, sagging and bleeding of a paste irregularly spread the line width of printed wiring patterns when a surface conductor layer is printed thick on the surface of a fired ceramic substrate. Consequently, it is difficult to print a fine wiring patterns sharp and thick. Furthermore, the prior art has proposed thick printing a surface conductor layer on the surface of a green sheet prior to firing and co-firing the printed surface conductor layer with the green sheet. However, the ceramic differs from the surface conductor layer in the shrinkage characteristic during the co-firing. This difference in the shrinkage characteristic results in occurrence of camber and/or waviness in the ceramic substrate, thereby reducing the yield and product quality.